Steady state channel select

ABSTRACT

In a wide band receiver, a pulse signal is directed through a plurality of narrow band channels. The pulse input is a short duration pulse of several cycles of the signal. The energy content in each of the narrow band channels is measured and compared to adjacent channels. During the steady state time, when the pulse signal is neither rising or falling, the energy differences between a narrow band channel being at the location of the incoming signal frequency and its adjacent channels would be maximum and exceed established thresholds. A comparison is then made with these established thresholds to produce logic signals demonstrating that the incoming is in an indicated narrow band channel. Where the signal is between two channels and in the crossover region, then the logic signals produced during the steady state pulse time would be indicative of substantially equal energy in the two narrow band channels having portions in the crossover region while the energy differences between each of those narrow band channels sharing the crossover region with their respective adjacent channels would be greater than the CCR levels. Logic signals indicative of these energy relationships are then processed to produce the signal indicative that the incoming signal is in the crossover region. The system operates to produce logic signals only during the steady state time of the pulse and is free of splatter which would otherwise prevent a clear and unambiguous indication of frequency.

FIELD OF THE INVENTION

This invention relates to a wide band receiver and the detection of asignal from among a plurality of signals in a densely packed widebandfrequency spectrum. It particularly relates to a detection and switchingsystem whereby a radio signal is detected and then used to develop aseries of logic signals to indicate the incoming signal frequency.

BACKGROUND OF THE INVENTION

Wideband receivers are associated with a large dynamic range. Associatedwith this dynamic range is frequency splatter. The incoming signal isselected from the wideband range typically by means of a filter.However, an incoming signal will cause frequency splatter extending overthe wideband and beyond it and causing energy to be injected into eachof the narrow band frequency selective filters. In wideband channelizedreceivers having contiguous channelized filters a splatter guardfunction providing an unambiguous indication of an inband signalfrequency represents a major element of cost. Prior techniques foranalyzing an inband signal involved wide and narrow filters andrepresented an investment of three filters per channel. An example isshown in U.S. Pat. No. 3,953,802 which uses the combination of wide andnarrow band filters for determining when a desired signal is in thenarrow band channel.

SUMMARY OF THE INVENTION

According to the principles of this invention a narrow band frequency isidentified within a wideband dynamic range by an efficient resolution ofthe incoming signal frequency. According to the present invention,instantaneous frequency analysis of signals in a wide dynamic range maybe accomplished by injecting the wideband signal input into a bank ofnarrow band filters (channels) and then providing selected activityinformation with regard to each channel output. The invention operatesby recognizing that energy splatter over all channels is significantlypresent during the rise and fall time of the pulse, but notsignificantly present during its steady state time. The splatter leveldoes not vary significantly with pulse carrier frequency and thetransition energy over all channels is substantially evenly distributed,falling off slowly from a channel as the carrier frequency is moved awayfrom the channel. Due to this splatter, introduction of a pulse signalinto a wideband receiver, causes the channels and their filtersconnected therein, to become active whether or not the pulse carrierfrequency is within the channel pass band. Consequently, filterassignment tasks and frequency discrimination are difficult if notimpossible. According to the principles of the invention, the inputsignal spectrum representing the wideband spectrum, is routed through abank of channelized narrow band filters. The bank contains narrow bandfilters which may be contiguous and with each channel defined by anarrow band filter having a pass band representing a part of thewideband input. The invention operates by comparing signal levels duringthe pulse steady state time as frequency selectivity is minimum duringthe pulse carrier rise and fall times when splatter is substantial.Therefore, the channelizer is designed to respond to the steady stateportion of the pulse carrier when splatter is less and the energy leveldifferences between adjacent channels is greater. Accordingly, thechannelizer will force a correct channel assignment or frequencyidentification in the presence of large amounts of splatter or ofsimultaneous signals when separated by at least one defined channel.

According to the inventive principles, the preferred embodimentcomprises a bank or set of narrow band filters defining channels whichare connected to receive a wideband frequency input. The output of thechannels are provided to a log amplifier bank which provides amplitudelevels related to the energy levels in each channel. In accordance withthe principles of the invention, energy levels within adjacent channelssignificantly differ during the steady state period of the pulse. Theoutputs of each of the channels are compared to each other and tochannel to channel reference levels to produce an output indication. Theindication is produced by a means which is nonresponsive to energy inthe narrow band filters in the period of greatest splatter during riseand fall times of the pulse carrier, but which is responsive during thesteady state portion of each pulse, when a reliable output indicationcan be produced. During the steady state pulse time, an indication ofthe energy level in each narrow band filter is provided by means ofseparate channel-related log video amplitude outputs and then bycomparing the log video amplitudes for related or adjacent channels.Comparison of the log video voltages removes the effects of splatter byproviding a residue signal which is an analog of the difference in powerlevels between channels. The residue signal is then compared to athreshold level or channel to channel ratio level (CCR) which producesan indication of channel pairs having a signal difference or energylevel difference exceeding the CCR. These CCR's are chosen to be greaterthan the channel to channel energy level differences during the edgetransition time or rise and fall time, with the result the channelizerdoes not produce an output and is forced to be nonresponsive during theedge transition times. In between edge transition time, after the risetime and before the fall time of the pulse carrier, during steady state,frequency detection may be accomplished.

According to the principles of the invention the CCR's may be chosen asclose as possible to the channel to channel energy level differencesduring the period of greatest splatter, but sufficiently greater thanthose energy levels to produce an unambiguous response in the systemduring the steady state time. As would be known by one skilled in theart where the comparators are made more sensitive, the CCR levelthreshold may be brought closer to the energy level differences actuallyexperienced during the periods of greater splatter. Additionally, itshould be recognized that where the term exceeded is used, it means thecrossing of one amplitude over another, such as the energy levelamplitude over the CCR level and may be in either of the two possibledirections without departing from the principles of the invention (i.e.the energy level may exceed the CCR by going higher in a positivedirection or in a negative direction.

In the preferred embodiment, a center channel or center channels arearranged with adjacent guard channels on each side. The amplified outputof the center channel is compared with the center channel's respectiveguard channel outputs. In the preferred embodiment, these comparisonsare energy difference comparisons.

The channel difference energy levels then are compared to the CCRlevels. The output of the CCR comparators are logic signals and areprovided to first and second logic trees which produce a first logicsignal to responsively define the frequency. The logic trees provide afirst frequency indication signal of whether an input signal is within acrossover range between adjacent channels or within a channel filtermidband frequency range.

Additionally, the crossover and midfilter logic indications may also beprovided to a third logic tree which has as a second input, energy levelindicative signals related to frequency and to energy level differencesbetween channels and produces a second logic signal responsive to theinput signal frequency. While the first logic signal is sufficient todefine the input frequency, it may be combined with the second logicfrequency indicative signal produced by the aforesaid channel to channelenergy comparisons and third logic tree to produce a third frequencyindicative logic signal.

Further, and according to the principles of the invention, the trailingedge of the incoming pulse may be used to inhibit processing during thepulse fall time. When the scalloping on a pulse's edge is deepest, logictree discriminator CCR gates and channel assignment comparators mayattempt a false assignment. The trailing edge blanking signal istherefore developed to guard against false CCR generation.

The channelizer logic shown herein may be used to provide channelactivity indications for purposes of spectrum analysis by frequency cellor channel. The logic shown herein produces a filter band signalindication within the dynamic ranges derived from the energy or envelopedetector performance.

As a first object of the invention, it time selectively produces afrequency indicative first logic indication response during that periodof the pulse when it is at steady state and produces a splatter-freesignal. This is accomplished for the case when the signal is within thecenter or midfilter range of a channel through a first logic tree andwhen in the crossover range of a channel through a second logic tree andfor other cases as described.

Further, and as a second object of the invention, a second logicindication may be produced by comparing the frequency dependent energylevels of the center and guard channels to each other through a third ordiscriminator logic tree to produce a second set of frequency dependentlogic signals. These second set signals may be used alone as in the caseof the first set of midchannel or crossover CCR logic signals to producea frequency indicative output or may be used by itself combined with thefirst set logic signals to produce a time-dependent frequency indicativeoutput.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wideband input and a bank of narrow band filter channelsfor receiving a pulse input.

FIG. 2 shows a filter response with respect to frequency.

FIG. 2a shows the amplitude vs. frequency response of three filtersarranged with a crossover region therebetween.

FIG. 3a-f show the resultant energy levels within a band of filters, inresponse to an incoming signal.

FIG. 4 shows a first logic tree for determining whether an incomingsignal is in a channel's midfilter range during the steady state pulseperiod.

FIG. 4a shows a second logic tree for determining whether an incomingsignal is in the crossover range between channels during the steadystate pulse period.

FIG. 5 shows a third logic tree used to determine the frequency of anincoming signal by directly comparing frequency responsive energy levelsin adjacent channels.

FIG. 6 shows in general form the number of elements for each first,second and third logic tree elements for a bank of N filter channels.

FIG. 7 shows by transfer characteristics, the ability of the inventionto identify frequency when a plurality of signals, shown as examples,are separated by an inactive channel.

FIGS. 8-11 show the frequency locations of two simultaneously occurringincoming signals separated by an inactive channel and represented by therespective transfer characteristics of FIG. 7.

Referring now to FIG. 1, the splatter problem may be referenced tosingle input pulse 101 shown as a 990 MHz signal having a pulse width of100 nanoseconds. The signal is provided on line 103 into wide badereceiver 105. The output of receiver 103 is supplied to a number offilter channels 107A through 107F each of which have a defined pass bandor channel width within the wide band frequency spectrum. As would beknown to those skilled in the art, the outputs of the channels are leastdistorted at the channel having a center frequency closest to the inputsignal frequency. As the object of the invention is to identify theincoming frequency within the broad band input, the inventionaccomplishes this object by identifying the energy content of thechannels excited by the input signal. The narrow band filter having afrequency most closely matched to the incoming signal frequency wouldhave the greatest energy. However, as explained, during pulse time andfall, the splatter causes simultaneously broad band uniform energydistribution over all of the separate channels so identification isdifficult if not impossible. This invention may accomplish its object bytime-related identification of energy levels within the narrow bandfilters.

In FIGS. 3a-3e, a number of possible cases are shown for the energydistributions within the channels 107A-G and the logical manner thoseenergy levels may be analyzed to provide a clear unambiguous indicationof the incoming signal frequency.

Referring now to FIG. 2, the frequency response of a narrow pass filterwithin a channel such a 107A may be shown where the center frequency isshown as fo, the lower crossover point being f×l and the high crossoverpoint being f×h. The crossover points are arbitrary designations and inthis case are shown as the -3 dB points in filter responsecharacteristic relative to the center frequency. Additionally, but notnecessary to the principles of the invention, the filter may becharacterized by its response at frequencies midway between the centerfrequency fo and the crossover points f×l and f×h shown here for exampleas the -1 dB point at the lower-upper quarter boundaries. As would beknown by those skilled in the art, the more selective the filter, thefaster the response falls off while the less selective the filter, theslower the response falls off relative to frequency. Typically filtersare designed for the response to fall off rapidly as shown by the dashlines, after frequency exceeds the crossover points in either direction.

Referring to FIGS. 3a-3f, the various cases are shown for the energydistribution over the narrow band filter channels, 107A to 107G ofFIG. 1. FIGS. 3a-3f show the energy distributions within each of thenarrow band filter channels A to G, with respect to a carrier appearingin one or more of the channels as shown. In FIG. 3a, the carrier fo isshown within a range of frequencies centered about the midpoint ofchannel D. As would be expected, the energy levels fall off in theadjacent channels on either side of D. It should be understood that forthe case of FIG. 3a and for cases 3b-3f, the amount of energydistribution in the adjacent filters would depend upon the carrierfrequency and its location relative to the center frequency of thefilter and the crossover points as shown with regard to FIG. 2. For thepurpose of explaining the invention, the energy distributions for FIGS.3a, 3b, 3d and 3e and 3f are shown for the steady state pulse period.The case for the time of greatest splatter, during the rise and falltime of the pulse is as shown in FIG. 3c. As can be seen, one of theadvantages of this invention is its ability to instantaneously compareenergy distributions and thus determine pulse steady state when asplatter-free indication may be obtained.

In FIG. 3b, the carrier fo is shown between filters C and D and locatedin the crossover range. It should be understood as in the mid-filtercase of FIG. 3a, that the crossover case is a range of frequenciescentered about the crossover point as defined for the adjacent filters,channels C and D. The energy distribution may be that as shown in FIG.3b where the carrier is in the crossover region. The energy levels wouldshift toward the right or higher frequencies as the carrier fo shiftstoward the right away from the crossover region and the energy levelswould shift toward the left as the carrier fo shifts in frequency towardthe left or toward a lower frequency.

In FIG. 3c, splatter or simultaneously broad band essentially uniform inenergy distribution is shown where the energy level differences betweeneach of the filters are small and more difficult to discern.

In FIG. 3d, the energy distributions are shown for two carrierfrequencies f₁, f₂, each located approximately at the midpoint of twononadjacent channels.

In FIG. 3e, the energy distributions are shown for the case of twosimultaneous signals, f₁, f₂, in nonadjacent channels with one signal ina crossover region and one signal being at a midchannel region.

FIG. 3f shows the energy distribution for the case of simultaneoussignals, f₁, f₂, at the respective crossover points of adjacentchannels.

Signal separation can be achieved by using filters to achieve inputspectrum frequency sampling over a designated bandwidth resolution. Inthe case of the preferred embodiment, the bandwidth is a -3 dB bandwidth resolution with regard to filter center frequency. According tothe principles of the invention, the channelizer logic detects channelenergy distribution over this 3 dB bandwidth resolution to determine theincoming signal channel frequency.

In the case of energy distribution in FIG. 3a, where the incomingcarrier is at or approximately near the center frequency or midchannelregion of the narrow band channel, then as shown, the energydistribution will be:

    D>C+D>E∴D=Real

For case 3b, where the signal is in the channel crossover region, thenthe energy levels would be:

    C≈D+C>B+D>E∴C or D=Real (not both)

In case 3c, where splatter produces essentially uniform energydistribution, then the energy distributions would be:

    A<B≈C≈D≈E>G∴none=Real

In the case of FIG. 3d, where simultaneous signals appears onnonadjacent channels, then the energy levels would be:

    C>B+C>D∴C=Real

    E>D+E>F∴E=Real

In the case of FIG. 3e, where simultaneous signals occur in nonadjacentchannels and with one signal being in a crossover region, then theenergy levels are:

    B≈C+B>A+C>D∴B or C=Real

    E>D+E>F∴E=Real

Finally, in the case of 3f where simultaneous signals occur in adjacentchannels at the crossover regions, then the energy distribution is:

    C≈D+C>B+D≯E∴C or D≠Real

    E≈F+E≯D+F>G∴E or F≠Real

The C≈D≈E≈F ambiguity is that either two signals are present with lessthan one filter separation or there is one signal in D≈E crossover or asplatter condition exists, thus the C, D, E and F channel responses areinhibited.

All of the six cases given can be logically decoded using the principlesof this invention.

The invention is now shown with regard to FIGS. 4, 4a, 5, and 6.

The channelizer uses the output of the channel or channels, in thecrossover case as in FIG. 3b, having the frequency response closest tothe incoming signal and uses the output of the guard channels, which areshown, in the preferred embodiment, to be the adjacent channels locatedon either side of the center channel or channels. The outputs of thecenter channel or channels and the guard channels are combined throughseparate logic trees to determine the incoming narrow band frequency.This is accomplished according to the principles of the invention byusing logic trees that are designed to time selectively respond to thesteady state pulse time during which incoming frequency may be measuredand which is designed to be nonresponsive to the pulse edge portion. Afirst logic tree relates the signal from the center channel or channelsand the adjacent guard channels to a CCR level to produce a firstlogical signal responsive to the incoming frequency and which is timeresponsive to the continuous or steady state portion of the incomingpulse when the pulse is splatter-free. The significance of producing apulse during the steady state pulse time, when the pulse issplatter-free can be seen with reference to FIGS. 3a, 3b and 3c. Duringthis splatter-free portion, the energy differences in each adjacentchannel are greater and are more easily measured. During the pulse time,splatter is greatest as in the case of rise and fall times, the energydistribution as in the case of FIG. 3c is less between the adjoiningfilter channels and more difficult to measure.

Referring now to FIGS. 4 and 4a, the first object of the invention isillustrated by log video outputs of a bank of N narrow band filtersillustrated with center frequencies from 950 MHz to 1030 MHz. The logvideo outputs shown are responsive to an input frequency with eachchannel having the center frequency and bandwidth of its respectivefilter output. The input signal in the example is shown at 990 MHz forFIG. 4 and at 1000 MHz in FIG. 4a, with the resultant log video outputsas shown for each channel.

The channel log video envelopes in FIGS. 4, 4a and 5 present energylevels at the outputs of each of the filters 107A-G, shown for examplein FIG. 1. The log video output may be a voltage level responsive toenergy and with the relative log video output levels of each channelbeing indicative of the incoming signal frequency and its distributionover the bank of narrow band filters, 107A-G, for example, (eachfilter's respective channel).

For each of the N channels, the channels adjacent to a respective one ofthe N channels represent guard channels and as may be seen, each of theN channels is a guard channel for adjacent ones of the N channels.

A first logic tree is illustrated in FIG. 4 with respect to a firstcenter channel for determining if the incoming signal is in the centerchannel midfilter range and for producing a first logic signalindicative thereof. A second logic tree is illustrated in FIG. 4a withrespect to center channels and adjacent channels for determining if anincoming signal is in the crossover region of the center channels forproducing a first logic signal indicative thereof.

As part of the first logic tree for determining whether the input signalis in the midfilter range, four difference amplifiers are connected tothe outputs of the center channel log video output and to the outputs ofthe adjacent guard channels. In the example of FIG. 4 shown, the filtersare all of a 20 MHz bandwidth and the central frequencies of each filterbeing given as 950, 970, 990, 1010 and 1030 MHz as stated. The pulsewidth of the input signal is approximately of 200 to 300 nanosecondsduration. The first logic tree comprises video difference amplifiers 113and 115 connected as shown, and comparators shown as 125, 127, eachconnected to the output of a respective video difference amplifier 113,115 and connected to a CCR level input. The CCR level for this case andthe following may be set at the level necessary and consistent with thefilter responses and the crossover points to be responsive to theenergies in the filters during the steady state pulse time and to benonresponsive during the edge portions of the pulse. For the generalcase filter as shown in FIG. 2, the threshold levels may be madeconsistent with the frequency response of the filters. As the frequencyresponse of the filter and its selectivity becomes less, the thresholdlevels would be changed accordingly.

The incoming signal produces log video outputs as shown. In the exampleshown, the incoming signal is approximately 990 MHz, the 990 MHz filteroutput is larger in comparison with the 1010 MHz and 970 Mhz filteroutputs, it being understood, the more removed a filter is from theincoming frequency, the less energy at its output, as shown in FIG. 3a.The difference amplifier outputs are shown as 111a, 113a, 115a, 117aandhave the highest level as shown for 115a and 113a resulting from thecomparison of the 990 MHz signal with the adjacent or guard channelsignals.

As the pulse input width is 200 to 300 nanoseconds, the output of thedifference amplifiers 113 to 115 are also 200 to 300 nanoseconds. TheCCR comparators are then used to compare these difference outputs 113aand 115a, with an established CCR level to produce a first logic signalresponsive to the steady state pulse period as shown in FIG. 4. This isaccomplished by setting the CCR's at a level greater than the channel tochannel level difference occurring when splatter is the greatest, asshown in FIG. 3c (during the edge portion of the pulse) and may be assmall as may be consistent with the sensitivity of the CCR comparators.In FIG. 4, a first logic tree is shown, for producing a first logicsignal indicative of the incoming signal being in the midfilter region,having a first comparator 125 connected through its negative input, tothe output of the video difference amplifier 113 which compares thecenter channel at 990 MHz with the next adjacent higher frequencychannel at 1010 MHz and produces a pulse of approximately 200 to 300nanoseconds. That pulse 125a corresponds to the steady state pulseperiod when the output of its corresponding video difference amplifier113 exceeds the CCR level. For the sake of explanation, it may beassumed that the CCR level is set at 6 dB. Comparator 123 does notproduce a pulse during the steady state time of the incoming signal, butdoes produce a pulse 123a during the pulse fall time of the incomingsignal when it peaks and is greater than 6 dB above the CCR. Comparator127 is connected to the output of difference amplifier 115 whichcompares the output of the center frequency filter and the next loweradjacent guard filter. It produces a logic pulse 127a when the output ofcomparator 127 exceeds the respective CCR by 6 dB (during the steadystate pulse time). A logic pulse is produced by comparator 129 at thefall time of the incoming signal, when the output of the comparatorfalls below the CCR. In FIG. 4 are also shown comparators 120, 121, and131 connected by difference amplifiers 110, 111 and 117 respectively tofilters having center frequencies of 1050 MHz, 1030 MHz, 1010 MHz, 970MHz and 950 MHz to show a partial expansion of the logic network forother center channels as explained below. Pulses such as 123a and 127aoccurring after the steady state pulse period may be used as an inhibitsignal.

As can be seen for the first logic tree shown in FIG. 4 where theincoming signal is within the midchannel range of a narrow band filter,then the first logic tree will produce a logic pulse, as shown at theoutput of gate 133, indicative of pulses 125a and 127a which indicatesthe incoming signal frequency is in the first logic tree center channelmidfilter region (990 MHz).

For the first logic tree of FIG. 4, used to detect when an incomingsignal is in the midfilter range of a channel filter, the filter havingthe midfilter range matching the incoming signal frequency range such as990 MHz, forms the center channel. The adjacent filters, such as 1010and 970 MHz, on either side, having higher and lower center frequenciesare guard channels whose outputs are compared with the center channeloutput and through the first logic tree to produce the indication whenan incoming signal frequency is within the midchannel range. As can beseen, the network of FIG. 4 can be expanded by adding filters above 1050MHz and below 950 MHz along with associated logic elements connectedthereto to construct additional first logic trees for other respectivecenter channels, which may serve a dual function as guard channels. Theoutputs of each designated center channel would then be combined withtheir respective guard channels (which may also serve as centerchannels) on either side thereof with respect to frequency to provide aclear unambiguous indication during the steady state pulse time when theincoming signal is in the midfilter region of that designated centerchannel. In summary, the first logic tree may be replicated for eachchannel to provide a midfilter first logic signal for its respectivechannel.

Part of the first logic tree of FIG. 4 such as the differenceamplifiers, may also be used in a second logic tree to detect signals inthe crossover regions of adjacent filters. The first and second logictrees are shown separately in FIGS. 4 and FIG. 4a, respectively, for thepurpose of explanation, it being recognized that common units may becombined as shown in FIG. 6. As can be seen, the first logic tree uses acenter channel and guard channels to decode the incoming signalfrequency. The object of this first logic tree as stated to produce alogic signal responsive to an incoming frequency signal in the midfilterregion and at a time when the signal is splatter-free during the steadystate. Where the incoming signal is located at the crossover region, at1000 MHz for example, and the energy levels within adjacent filterchannels are as shown with regard to FIG. 3b, then the output ofrespective video difference amplifiers of FIG. 4 would be as shown inFIG. 4a. FIG. 4a shows the signal levels out of the differenceamplifiers shown in FIG. 4 when the incoming signal is at a crossoverregion, 1000 MHz, for example.

As shown in FIG. 3b where the signal is in a crossover region, theenergy in adjacent filters such as C and D in FIG. 3b is approximatelyequal. The output of the difference amplifiers, 111, 113 and 115 are asshown by the wave forms 111a, 113a and 115a. During the steady stateperiod of the incoming pulse, when the energy distribution in the centerchannels is as shown in FIG. 3b, the outputs for the differenceamplifiers are substantially zero for 113 (113a), low for 111 (111a) andhigh for 115 (115a). The CCR comparators, forming the second logic tree,are 153, 155, 157, and 159. It should be understood that while aseparate set of comparators is shown for the case of the second logictree for crossover, comparators common to FIGS. 4 and 4a may be usedinterchangeably, as would be known to those skilled in the art. When theincoming signal (1000 MHz, for example) is in the crossover region oftwo adjacent filter channels, such as the filter channel having a centerfrequency of 990 MHz and the filter channel having a center frequency of1010 MHz, the energy levels therein are equal and their difference iszero as shown by the output 113a of difference amplfier 113. The outputsof the other difference amplifiers associated with the respective guardchannels are 111a and 115a as shown for amplifier 111 and 115. When theoutput is substantially zero, as for 113 and where that output is lessthan the CCR level of the comparators shown for 155 and 157, then thecomparator outputs 115a and 157a are low as shown. For differenceamplifiers 111 and 115 connected between adjacent center channel filtersand respective guard channels located on each side of the centerfrequency, such as 1030 MHz and 990 MHz channels, the comparator willproduce pulses 153a and 159a having a time duration corresponding to thesteady state portion of the incoming signal. For a signal in thecrossover region, the logic signals 153a, 155a, 157a and 159a of asecond logic tree may be combined as shown in an AND gate 160 to producea first logic signal 160a indicative of a signal in the crossoverregion. The term "first logic signal" is used interchangeably for amidfilter or crossover indication as the case may be. As in the case ofFIG. 4, the respective crossover center channels of FIG. 4a (1010 and990 MHz) are combined with guard channels (1030 and 970 MHz) or channelsadjacent the respective crossover center channels in a second logic treeto process the output of the video difference amplifiers. The centerchannels, 1010 MHz and 990 MHz, in the case of crossover region, containsubstantially the same energy. The energy in each adjacent or guardchannel to either side of the center channels is less as the incomingsignal frequency is further away from the center frequency of thesechannels. In the example shown in FIG. 4a, the logic signals shown areproduced for an incoming signal of 1000 MHz located in the channelcrossover range of 995 to 1005 MHz between the 990 MHz and 1010 MHzchannels. However, it should be understood that as the selectivity ofthe filters decreases and the response of the filter over frequencyincreases, the channel boundary will shift in toward the center of thesame CCR level. However, the crossover region of 995 to 1005 may beexpanded to 992 to 1008 also by increasing the CCR level requiring agreater difference in channel response at frequencies closer to thecenter frequency of the filters. As in the case of the midfilter firstlogic tree of FIG. 4, the crossover second logic tree of FIG. 4a may bereplicated for each crossover frequency region and respective channels.

Then as can be seen and according to the principles of the invention,the channelizer develops a signal indicative of the incoming signalfrequency and related to a narrow band filter channel in a first case bycomparing energies in a center channel in the midfilter case, or centerchannels in the crossover case, with adjacent or guard channels and byfurther processing signals representing that comparison through a firstor second logic tree respectively to produce a first logic signal todefine the incoming frequency during the steady state pulse time. In thecase of signals appearing within the midfilter range of a channel, theoutputs of that center channel, and the two adjacent or guard channelson either side of the center of that channel are processed to produce afirst logic signal responsive to the signal frequency during the steadystate portion of the pulse. In a similar manner when an incoming signalis located at the crossover region between two adjacent center channelfilters, the guard channel filters are on either side of the centerchannel filters. The outputs of the center channels and the guardchannels are then processed as shown in the second logic tree of FIG. 4ato produce a first logic signal indicative of signal frequency in thecrossover region between adjacent filters, during the steady stateportion of the incoming pulse and during the splatter-free time. Thelogic signal is produced by comparing channel to channel energies withpredetermined threshold levels, shown as CCR's to produce an outputresponsive to the steady state incoming signal pulse period, anduncontaminated by the energies generated by the pulse edge portions.

As the object is to define the incoming signal frequency, the inventiveprinciples are described with reference to such a result. However, itshould be understood that using the inventive principles, the signalscan be combined to make the result more definite. Additional logic canbe added and/or filter sensitivity can be increased to make the logicsignal more selective and related to a narrower band of incomingfrequencies.

Further, according to the principles of the invention, and as shown inFIG. 5, a second frequency responsive logic signal may be developed by athird logic tree. These signals are developed from the frequencyresponsive energies in each channel and use direct channel to channelcomparisons instead of comparison to channel to channel ratio levels(CCR).

In FIG. 5, outputs of the narrow band filter channels are provided aslog video output to difference amplifiers 161 and 163 which compare theenergy in a center filter channel with that of two adjacent or guardchannels on either side of the center channel. The single channeldiscriminator of FIG. 5 may be used alone or with the CCR gate outputsof the first and second logic trees of FIGS. 4 and 4a to provide analternate second frequency logic signal indication of incoming signalfrequency and to produce a signal occurring and resulting from thecontinuous or steady state portion of the input pulse when the signal issubstantially splatter-free. In this way, the pulse output of the CCRgating ensures that the frequency response pulse output of the thirdlogic tree frequency discriminator in FIG. 5 is produced only during thesteady state portion of the incoming pulse.

The single channel discriminator comprised by the third logic tree inFIG. 5 is responsive to energy levels within a center and adjacent orguard filters to determine the incoming signal frequency. The thirdlogic tree of FIG. 5 discriminates by comparing the relative amplitudesof the above stated energy levels, and is able to produce a frequencyoutput signal indicative of an responsive to those relative amplitudes.By itself it produces independent signals and may be further combinedwith the output of the CCR gates of the first and second logic trees toproduce a logic signal determinative of the incoming signal frequency.Whereas the CCR gates produce a signal responsive to the pulse steadystate, the discriminator of FIG. 5 produces amplitudes related to theresponse of the channel filters during the entire period of the inputpulse. As it responds to the energy in the filters during the steadystate as well as the rise and fall time, it produces an output signalover the pulse period. The discriminator as stated uses a center channelfilter and two adjacent or guard channel filters. Unlike the CCR gate,no thresholds are used. The relative energy levels within the centerchannel on the adjacent guard channels are compared in differenceamplifiers which produce first and second signal set intermediatesignals shown as "ESS" curves (161a and its inverse 161b, 163a and itsinverse 163b), and whose amplitudes vary above and below a median level,zero, for example, over the frequency range of its respective channelfilter. These relative amplitudes represent intermediate signals whoseamplitudes are related to frequency and which may be further processedby comparing the amplitudes to each other to produce a frequencyresponsive output. In the third logic tree shown in FIG. 5, thecomparator 171 is shown comparing the frequency responsive amplitudeoutput of the center channel and the adjacent upper frequency or guardchannel, or first signal set amplitudes 161a and 161b. Comparator 173 isshown comparing the amplitudes of a signal within the first signal set(161b) with a signal in the second signal set (163b). The comparator 175is shown producing a logic signal resulting from the comparison of thesecond signal set amplitudes 163a and 163b. It should be understood,however, that the logic connections as shown may be changed to produceany of a combination of logic signals desired and which may beinterpreted as the correct frequency. In the preferred embodiment andaccording to the principles of the invention, the frequency responsiveamplitudes derived from a comparison of frequency responsive energylevels in the filters are again compared to produce logic signals whichmay go high or low depending upon the changes in the amplitudes producedby the incoming frequency and the relative amplitudes at the input tothe comparators. Comparator 173, for example, produces a logic signalbased upon the relative excursion of the two amplitudes of the firstsignal set signal 161b and second signal set signal 163a. It can be seenthen as the input signal changes, the comparators 171, 173 and 175 willproduce changing outputs responsively. For example, in the preferredembodiment as shown, comparator 171 has an inverting output which goeslow at 1000 MHz and high at 1030 MHz. Comparator 173 has a noninvertinginput which goes high at 950 MHz and low at 990 MHz. It also has aninverting output which goes low at 950 MHz and high at 990 MHz oppositeto the output of the noninverting output. Comparator 175 has anoninverting output which goes high at 980 MHz and low at 1020 MHz.

When, for example, the incoming signal is in the midfilter range of thecenter channel or from 985 MHz to 995 MHz, a pulse output will beproduced at the noninverting output of comparator 173 and the invertingoutput of 173 and at the output of 175.

These signals at the output of comparators of 171, 173 and 175 may befurther combined with the outputs of the first and second logic trees ofFIG. 4 and FIG. 4a to provide pulses of greater frequency resolution.For example, the output of comparator 171 may be supplied to an AND gate181 having as its second input the output 160a of the AND gate 160 ofthe crossover CCR gate of the second logic tree shown in FIG. 4a. Theinverting and noninverting output of amplifier 173 may be provided totwo separate AND gates (183, 185) which have as second enabling inputs,the output 133a of the midfilter CCR gate first logic tree shown in FIG.4. The output of the comparator 175 may be supplied to an AND gate 187which has a second input, the output (162a) of a crossover CCR gatefirst logic tree (not shown) producing an output when the incomingsignal is at 975 to 985 MHz.

Similarly, to input 160a to AND gate 181 (from AND gate 160), the inputsignal 162a may be applied to AND gate 187 responsive to an input signalin the crossover range 975 to 985 MHz. As may be seen, the second logictree of FIG. 4a would be duplicated for the crossover channels 970 and990 MHz.

The first and second logic trees could be built for each of the Nchannels and to detect a signal located in a channel midfilter region orcrossover regions between two channels. Accordingly, by duplicating thefirst logic tree of FIG. 4 and second logic tree of FIG. 4a for eachcenter channel region, as for example 990 MHz in FIG. 4, or crossoverregion, as for example 995 to 1050 MHz in FIG. 4a, the logic trees thenwould provide first logic signals of the input frequency, whether in amidfilter or crossover region at the pulse steady state time. As shownin FIG. 5, the first logic signals could be combined with the secondlogic signals of the third logic tree to produce frequency indicativesignals derived from the energy levels in the N filters during the timeduration of the steady state input pulse when splatter is minimized andthe energy differences in the separate N channels is maximized.

Additionally, common units may be combined to reduce the number ofcomponents as shown in FIG. 6, where the total number of channel is N.Accordingly, an intercept channelizer block diagram is shown in FIG. 6wherein for each N channel, there are N+4 filters (two filters on eitherside of the center N channel) N+4 log amps (for the amplification of theenergy levels within the channel filters), N+3 video differenceamplifiers (required to amplify and detect the energy difference levelswithin each of the channel filters). Also, in parallel with the CCRcomparator logic tree are two (N+3) discriminator comparators and four(N+3) discriminator gates comprising the discriminator third logic tree.

The meaning of FIG. 6 and its representation of the number of componentsrequired for each of the N channels is as follows. The diagrams of FIGS.4, 4a and 5 show the logic trees for a center channel or center channelswhere logic trees as shown in FIGS. 4, 4a and 5 are duplicated for eachof the N channels. As shown, FIG. 6 represents the first, second andthird logic trees receiving information from the N channels and theassociated guard channels. As would be known to those skilled in theart, it would not be necessary to build separate logic trees for each ofthe channels as the incoming signal frequency and energy levels woulddetermine which logic trees would be used. In paraticular those logictrees connected to the center channel or crossover channels of theincoming signal and the adjacent guard channels would have sufficientenergy to produce a frequency indicative logic signal. Therefore, thelogic trees may be connected to receive the output of two or more of theN channels and the respective guard channels where signals in thosechannels would not present a conflict in the use of the associated logictrees.

Referring now to FIG. 2a, it may be seen how for adjacent channels, themidchannel and crosschannel regions may be established by using CCR'sfor the thresholds in the CCR gating networks of FIGS. 4 and 5. Asshown, as the input frequency changes, the energy level within each ofthe channels will change and the amplitude of the channel log videooutput will change. The video difference amplifiers in the CCR gatingnetwork will then produce the difference between the channel log videooutputs at any particular frequency. These outputs will be compared inthe CCR comparators. By selecting an appropriate CCR threshold level ineach comparator, the state of the comparator can be forced to changewhen the difference between the output of the energy levels in theadjacent channels and the corresponding difference output of the videodifference amplifier falls above or below the threshold. In the exampleshown here, the threshold level is shown as 6b. A crossover regionbetween the filter channel having center frequency f₁ and the filterchannel having frequency f₂ is then shown in the frequency band betweenfx₁₂ and fx₂₁. At those levels, the difference between the response atthe output of filter channel f₁ and the output of filter channel f₁ isless than 6 dB and does not increase to 6 dB until the incomingfrequency is greater than fx₂₁. Similarly, the setting of the thresholdsas shown also defines the midchannel region, for example, for thechannel f₂ the midchannel region would be between fx₂₁ and fx₂₃. Thecrossover and midchannel regions can be varied in any predeterminedfashion then by varying the threshold levels and thereby moving themidchannel and crossover region frequency ranges.

Then, as can be seen and according to the principles of the invention,the channelizer develops a signal indicative of the incoming signalfrequency and related to a narrow band filter channel by processing thesignal through the filter channel comprising that incoming frequency andby processing it through guard channels also comprising filters havingdefined narrow band widths. In the case of signals appearing within themidfilter range of a logic tree center filter channel, the outputs ofthat filter channel, and the two adjacent or guard channels on eitherside of that channel comprising the frequency are processed to producelogic signals responsive to the steady state portion of the pulse. In asimilar manner, when an incoming signal is located in the crossoverregion between the center frequencies of two adjacent filters, thecenter channel filters, on either side of the crossover region areprocessed as well as that of the guard filters with respect to thecenter channels and on either side of the center channels. The outputsof the channels tuned to the signal and the guard channels adjacent andadjoining thereto are then processed as shown to produce logic signalsduring the steady state portion of the incoming pulse and during thesplatter-free time when the amplitude difference representing the energylevels in the filters is efficiently different so that logic signals maybe produced using appropriate threshold levels.

Further, according to the principles of the invention and as shown inFIG. 5, frequency discrimination logic signals are developed which maybe combined with the CCR gating signals to produce a signal indicativeof the incoming frequency normalized to the band width of the respectivefilters.

As the object is to define the incoming frequency in as narrow a band aspossible, the application of the principles herein are described withregard to such a reasonable result. However, it should be understoodthat signals may be further combined to make the result less ambiguousor the frequency indication may be produced relative to a wider band ofincoming signals or additional logic can be added and filter sensitivitycan be increased to make the logic signal more selective and related toa narrower band of incoming frequencies.

Referring now to FIG. 6, the intercept channelizer block diagram isshown for the generalized case where the concept of establishing narrowband pass filter channels within a wideband pass input and identifyingthe correct incoming frequency by the energy distribution producedthroughout a center channel matched to the incoming frequency on guardchannels and comparing the energy levels in the center channel and theguard channels to determine the frequency of interest corresponding tothe incoming signal during the steady state period of the incomingsignal when it is most splatter-free is shown. As would be obvious toone skilled in the art, elements can be used in common by a centerfrequency and guard channel tree. However, the generalized case will bemade with respect to a channelizing input having N channels.

Referring now to FIG. 2a, it may be seen how the midchannel and crosschannel regions may be set by using the thresholds in the CCR gatingnetworks in FIGS. 4, 4a and 5. As shown, as the input frequency changes,the energy level within each of the channels will change and theamplitude of a channel log video output will change. The videodifference amplifiers on the CCR gating network will then take thedifference between the channel log video outputs at any particularfrequency. These outputs will be compared in the channel to channelratio comparators. By selecting an appropriate threshold level in eachcomparator, the state of the comparator will change when the differencebetween the output of the energy levels in the adjacent channels and thecorresponding difference output of the video difference amplifier fallsbelow the threshold. In the example shown here, the threshold level isshown as 6b. A crossover region between the filter channel havingcentral frequency f₁ and the filter channel having frequency f₂ is thenshown with the frequency band between fx₁₂ and fx₂₁. At those levels,the difference between the response at the output of filter channel f₁and the output of filter channel f₁ and the output of filter channel f₂is less than 6 dB and does not increase to 6 dB until the frequency isgreater than fx₂₁. Similarly, the setting of the thresholds also definesthe midchannel region. For example, in the channel f₂ between fx₂₁ andfx₂₃. The crosschannel and midchannel regions can be varied in anypredetermined fashion then by varying the thresholds.

The principles of this invention may also be used to determine thefrequencies of incoming signals simultaneously appearing in the widebandinput and separated by one or more inactive channels. As shown in FIGS.3d, 3e and 3f, simultaneous signals may appear centered in separatechannels as shown in FIG. 3d or with one signal appearing centered in achannel and the other signal appearing in the crossover region as shownin FIG. 3e or with the two simultaneous signals appearing in thecrossover regions of separate channels as shown in FIG. 3f. In the caseof signals separated by one inactive channel, the distributions ofenergy levels in each channel can still be compared and measuredaccording to the principles discussed so that the energy centroid may belocated with respect to each incoming signal. However, splatter fromeither signal may extend over several channels and one signal splattermay momentarily blank the other. This phenomenon, however, as explained,will occur only during the rise and fall time of the adjacent signal.When simultaneous signals are separated by several channels, thestrongest signal may suppress the weaker signal.

When simultaneous signals occur, the centroid location of the frequencyfor each incoming signal may be measured with the first, second or thirdlogic trees as described above. The ability to detect the signals willvary with the frequency spacing between the simultaneous signals and theamplitude difference. As the amplitude difference increases, the abilityto detect the frequency of two incoming simultaneously-appearing signalsis made more difficult. However, as the frequency separation increases,the ability to detect the incombing frequency signals is made relativelyeasy as such that signals of greater amplitude separation may bedetected at wide frequency separations as will be shown and describedbelow.

FIG. 7 is a plot of channelizer performance for two signals (f₁, f₂)anywhere within its signal separation frequency and amplitudecharacteristics.

The four cases shown in FIG. 7 are for respective examples of signalseparation as represented in FIGS. 8, 9, 10 and 11. It should beunderstood, however, that the four cases as defined therein are shownfor illustrative purposes, and the frequency spectrum for the incomingsignals may be shown in other ways and specifically with other definedfrequency locations for the signals of interest. However, these examplesshown in FIGS. 8, 9, 10 and 11 and the characteristic for each of thefour examples shown in FIG. 7, chosen to explain the inventive principlewill illustrate the principle of the invention and its ability toidentify frequency centroids for simultaneously-appearing signalsseparated by more than one inactive channel.

To assist in the explanation of this foregoing concept, reference ismade to FIG. 2 wherein the bandpass for a filter is shown divided intoquarters, with the crossover frequency points fxl and fxh being theextremes and with a lower half defined by the center frequency and thelower crossover point and an upper half defined by the center frequencyand the upper crossover frequency fxh. The quarter points are definedmidway between the crossover points and the center frequency in eachhalf, the lower and upper half respectively. In the example of the 20MHz band pass filter of the preferred embodiment, the quarter pointswill occur 5 MHz from the crossover points and the center frequencypoints. For the purpose of explaining the manner in which the inventioncan identify simultaneously-appearing signals separated by an inactivechannel, the chosen illustrations of FIGS. 8, 9, 10 and 11 show twosignals f₁ and f₂. For the purpose of explanation, f₂ is always shown asthe higher frequency signal and appearing in channel fc₃ at the -1/4point (see FIG. 2) midway in the lower half of the frequency response ofchannel fc₃ and moves upward in frequency from that point. The lowerfrequency signal f₁ is shown at different locations within the lowerfrequency filter channel fc₁ and is at the upper quarter in FIG. 8, isat the midway point in FIG. 9, is at the lower quarter point in FIG. 10,and is at the lower crossover point at the lower frequency end of thelower half of channel fc₁. For purpose of explanation, it is assumedthat the signal f₁ does not change in frequency and signal f₂ increasesin frequency. FIG. 7 shows the range in which the preferred embodimentcan distriminate between two simultaneous signals separated by aninactive channel using the 20 MHz filter channels and the 6 dB CCR's asdescribed. However, it should be understood that the response shown inFIG. 7 is merely illustrative of the preferred embodiment and should notbe thought as limiting the ability of the invention to identifysimultaneous signals. As would be known to those skilled in the art, theability to identify simultaneous signals can be increased or decreaseddepending upon the elements chosen and the selectivity and the responseof those elements.

Referring now to FIG. 7, the ordinate shows the power of the secondsignal of frequency f₂ relative to the first signal of frequency f₁ foreach of the incoming signals shown in FIGS. 8, 9, 10 and 11. For thepreferred embodiment, and for the case shown in FIG. 8 where the lowersignal f₁ is at the upper quarter point in channel fc₁ and the upperfrequency signal f₂ is at the lower quarter point, then for 20 MHzbandpass channels, the signals would be separated by a minimum of 30MHz. At 30 MHz, the preferred embodiment may detect the frequencies ofthe simultaneous-appearing signal when the second signal power relativeto the first is less than ±8 dB. As shown, where the second signal powerrelatie to the first is greater than ±8 dB or where the first signalpower relative to the second is greater than ±8 dB, the second signalpower suppresses the first signal or the second signal is suppressed bythe first signal. For FIG. 8, as frequency increases from 30 MHz, thepreferred embodiment is able to identify the frequency of the incomingsignals where for any frequency separation between f₂ and f₁ as given bythe abscissa, the ratio of f₂ signal power relative to that of f₁ isbetween the upper and lower curves denominated as FIG. 8. For example,where f₂ increases in frequency such as where the separation is 60 MHz,then where the second signal power relative to the first is within ±3dB, the system can identify the frequencies f₁ and f₂.

Whereas shown in FIG. 9, f₂ is at the -1/4 point in filter channel fc₃and f₁ is at the midpoint of channel fc₁, the separation will be 35 MHz,for the 20 MHz channels. In that case, at 35 MHz the system will be ableto identify the two frequencies when the second power relative to thefirst power is ±8 dB. As f₂ increases in where the relative power ratiois on or between the two curves shown as FIG. 9. Accordingly for FIG.10, where f₂ is at the -1/4 point in channel fc₃ and f₁ is shown at the-1/4 point in channel fc₁ and separated by 40 MHz (where the channelsare 20 MHz wide), then for a relative ratio of f₂ to f₁ of ±8 dB, thepreferred embodiment will be capable of identifying the second and firstsignal frequencies (f₂, f₁) at 40 MHz and, as f₂ increases, for allpoints along the curve shown as FIG. 10 and for points in between thecurves shown as FIG. 10.

Similarly for FIG. 11, where f₂ is as shown, and f₁ is at the lower edgeof the lower half of channel fc₁ and separated from f₂ by 45 MHz, (forthe case of 20 MHz wide channels), then where the power ratio of f₂ tof₁ is ±8 db, and at 45 MHz separation, the system will be able toidentify the two signals and at all points along the curve and betweenthe curves labeled FIG. 11.

It should be understood, however, that by making the components moreselective, and by changing the resolution of the center channels, thesystem according to the principles of the invention may have a greaterability to identify incoming signal frequencies separated by an inactivechannel, for smaller separations in frequency and for greater relativepower ratios of second to first signal power.

I claim:
 1. A method for determining the frequency of a narrow bandsignal received at the input of a wide band receiver having a pluralityof N narrow band channels comprising the steps of:(a) measuring theenergy levels produced by said received signals in a plurality of Nchannels within said wide band; (b) comparing the said energy levelswith guard channels adjacent respective N channels and producing firstinput signals indicative of N channel to guard channel energydifferences; (c) establishing channel to channel ratio levels (CCR's) ina logic tree indicative of expected energy level differences betweensaid N channels and respective guard channels, when said narrow bandsignal is in its steady state condition; (d) comparing said first inputsignals with said CCR's to produce logic signals identifying said firstsignals which exceed said CCR's; and (d) responsive to said logicsignals, producing an indication of the frequency of said receivednarrow band signal.
 2. The method of claim 1, wherein said step (c)includes the step of3 establishing said CCR's to be less than the saidexpected energy level differences between said narrow band channels andsaid respective guard channels when said narrow band signal is rising orfalling.
 3. The method of claim 1, wherein said step (d) includes thestep of:producing said logic signals to identify a said narrow bandchannel, of said N channels, having energy level differences betweensaid narrow band channel and its guard channels exceeding said CCR's,and processing said logic signal in said logic tree to produce a logicsignal indicative of said received signal being in a mid-filter range ofa narrow band channel within said N channels.
 4. The method of claim 1,wherein said step (d) of producing said logic signals includes the stepof:producing said logic signals to identify a pair of adjacent narrowband channels, of said N channels, having an energy level differencetherebetween which does not exceed said CCR's and having energy leveldifferences between each of said pair of said adjacent narrow bandchannels and their respective guard channels which exceed said CCR's, insaid logic tree to produce a logic signal indicative of said receivedsignal being in a crossover region between the two said adjacent narrowand channels.
 5. The method of claim 1 wherein step (c) includes thestep ofdefining mid-filter regions of said narrow band channels andestablishing CCR's indicative of the energy level differences betweensaid narrow band channels and their respective guard channels when saidreceived signal is in a respective mid-filter region of said narrow bandchannels; said step (d) includes the step of comparing the energy leveldifferences between said narrow band channels and their respectivehigher frequency guard channels with said CCR to produce a firstindication when said CCR is exceeded; comparing the energy leveldifference between said narrow band channels and their respective lowerfrequency guard channels with said CCR to produce a second indicationwhen said CCR is exceeded; and said step (e) includes the step ofproducing a logic signal indication of said received signal being in asaid narrow band channel mid-filter range in response to said first andsecond indications.
 6. The method of claim 1, wherein said step (c)including the step ofdefining the crossover region of said N channels;establishing a CCR indicative of the energy level differences betweentwo adjacent narrow band channels and establishing a CCR indicative ofthe energy level differences between each of the said two adjacentnarrow band channels and their respective guard channels, when saidreceived signal is in the crossover region of said two adjacent narrowband channels, one guard channel being higher in frequency and one guardchannel lower in frequency, the two adjacent narrow band channelfrequencies; said step (d) including the step of comparing the energylevels in said two adjacent narrow band channels to produce a firstindication when said energy level difference does not exceed said CCRand comparing said energy levels between said two adjacent narrow bandchannels and their respective guard channels to produce a secondindication when said energy levels exceed said CCR's; and said step (e)including the step of producing a third indication indicative of saidincoming signal being in said crossover region between said two adjacentnarrow band channels in respose to said first and second indications. 7.A system for determining the frequency of a narrow band signal receivedat the input of a wide band receiver having a plurality of N narrow bandchannels comprising:(a) measuring means, for measuring the energy levelsproduced by said received signal in a plurality of said narrow bandchannels within said wide band; (b) comparing means for comparing thesaid energy levels with guard channels adjacent respective said narrowband channels and for producing first input signals indicative of narrowband channel to guard channel energy differences; (c) logic tree meansincluding channel to channel ratio level (CCR) comparing means forcomparing said first input signals with channel to channel ratio levels(CCR's) indicative of expected energy level differences between saidnarrow band channels and respective guard channels, when said narrowband signal is in its steady state condition, and including generatingmeans for producing logic signals identifying respective ones of saidfirst signals which exceed said CCR's; and (d) indicating meansresponsive to said logic signals for producing an indication of thenarrow band location and frequency of said received narrow band signal.8. The systems of claim 7, wherein said CCR comparing means includesmeans for establishing said CCR's to be greater than less than the saidexpected energy level differences between said narrow band channels andsaid respective guard channels when said narrow band signal is in itssteady state.
 9. The system of claim 7, wherein said CCR comparing meansincludes means for establishing said CCR's and includes identifyingmeans for identifying those narrow band channels having an energydifference between it and said respective guard channels exceeding saidCCR's.
 10. The system of claim 9, wherein the said CCR comparing meansincludes generating means responsive to said identifying means forproducing a logic signal responsive to said identifying means andindicative of the received signal being a mid-filter range of said Nchannel.
 11. The system of claim 9, wherein said means for comparing,comparing the energy level differences between said narrow band channelsand their respective guard channels having a higher frequency to producea first indication when said CCR is exceeded;said means for comparing,comparing the energy level differences between said narrow band channelsand their respective guard channels having a lower frequency to producea second indication when said CCR is exceeded; and said logic tree meansincluding a first logic tree means, responsive to said first and secondindication for producing a logic signal indication of said receivedsignal being in a narrow band channel mid-filter range.
 12. The systemof claim 9, wherein said means for comparing, compares the energy levelsin two adjacent narrow band channels to produce a first indication whensaid energy level differences does not exceed said CCR and compares saidenergy levels between said two adjacent narrow band channels and theirrespective guard channels to produce a second indication when saidenergy levels exceed said CCR; andsaid logic means responsive to saidfirst and second indication producing a third indication indicative ofsaid incoming signal being in a crossover region between said adjacentnarrow band channels.
 13. The system of claim 7, wherein said CCRcomparing means includes means for comparing said first input signalsindicative of said narrow band channel to respective guard channelenergy differences and identifying means for identifying a narrow bandchannel pair having an energy level difference between each of said pairof said N channels and their respective guard channels not exceedingsaid CCR's.
 14. The system of claim 13, wherein said CCR comparing meansincludes generating means responsive to said identifying means forproducing a logic signal indicative of said received signal being in acrossover region between the two said identified N channels.